13Is the answer... 15 ?
Please can anyone please show me a detailed explanation of the method to find the co-eff. of x11 in the expansion :
(1+x+x2+x3+.......+x6)3
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5 Answers
19coeff of x11 in (1+x+x2+......x6)3
=coeff of x11 in {(1-x7) /( 1-x)} 3......(summation of gp series)
=coeff of x11 in (1-x7)3 (1-x)-3
=ans =33
1g.p summation formula:
(1-x7)3(1-x)-3 (1-x)-3=1+a1x+a2x2+..........∞
ar=(r+1)(r+2)/2
hence x11 coefficient is coming on two combination x7.x4 and x11
so answer is 78-45(a11-3a4) =33